(1) Field of the Invention
The present invention relates to image formation apparatuses, and in particularly to technology for preventing the amount of remaining toner in a hopper from being misdetected due to magnetic noise in the case of using a reed switch.
(2) Description of the Related Art
Electrophotographic image formation apparatuses form an image by first transferring a toner image, which has been obtained by developing an electrostatic latent image on a photosensitive drum by using a developer, onto a recording sheet, and then fixing the toner image by heat. The developer is supplied with toner from a detachable toner cartridge. Here, on the way of being conveyed to the developer, the toner is temporarily stored in the hopper.
FIG. 13 shows a mechanism for supplying the developer with toner from the toner cartridge, adopted in an image formation apparatus pertaining to prior art. As shown in FIG. 13, a toner cartridge 1301 has a cylindrical shape. The inner surface of the toner cartridge 1301 is provided with a protrusion that spirally extends along the surface. The toner cartridge 1301, which is detachable, is attached to a container holder 1302. When it is presumed that all the toner contained in the toner cartridge 1301 has been consumed, the toner cartridge 1301 is detached from the container holder 1302, and is replaced with a new one.
The toner cartridge 1301 has a toner outlet 1303. The end of the toner cartridge 1301 is connected with a drive shaft (not depicted). The toner cartridge 1301 is rotated by drive force from a motor 1304 via a gear wheel 1305. Due to the rotation, the spiral protrusion on the inner surface of the container conveys the toner. Thus, the toner is supplied to a hopper 1306 via the toner outlet 1303.
The hopper 1306 contains a stirring fin 1307 that is made from a thin elastic plate. The stirring fin 1307 is caused to slowly rotate along the inner surface of the hopper 1306, which prevents the toner from adhering to the inner surface, or being solidified.
A supplying screw 1308 is provided near the bottom of the hopper 1306. The supplying screw 1308 is rotated by drive force from the motor 1309. The supplying screw 1308 supplies the developer 1310 with an appropriate amount of toner, according to the amount of the toner stored in the developer 1310. The developer 1310 supplies toner onto the surface of a photosensitive drum 1311 in order to form a toner image.
To supply the hopper 1306 with an appropriate amount of toner from the toner cartridge 1301, it is necessary to measure the amount of the toner stored in the hopper timely.
In view of this necessity, various technologies have been developed. The technologies include: providing a piezoelectric sensor within the hopper and monitoring a signal that changes according to the amount of the toner contacting the surface of the sensor; using a photo interrupter to monitor whether the amount of remaining toner is greater than a given level; and using a reed switch to detect the position of a magnet attached to a detection plate placed on the surface of the toner. For example, see Japanese Patent Application Publication NO. 2001-100508.
In recent years, the demand for full-color, low-cost, and downsized image formation apparatuses has been increasing. In particular, a full-color image formation apparatus requires toners for many colors, and thus it is necessary to provide a hopper for each of the colors. This means that it is necessary to further downsize each of the hoppers, compared to black-and-white image formation apparatuses which require only one hopper.
In this regard, to minimize the cost, it is preferable to use a photo interrupter or a reed switch, because piezoelectric sensors mentioned above cost relatively expensive.
However, photo interrupters need a cleaner device to clean their light-transmissive windows, because dirt on the window deteriorates the detection accuracy. Such a cleaning device inherently needs to have durability at the same level as the image formation apparatus, which is a hindrance to the production of long-life and downsized image formation apparatuses with a low price.
The cases of using a reed switch also have a specific problem. A full-color image formation apparatus requires toners for many colors, and thus it is necessary to provide a hopper for each of the colors. This means that it is necessary to further downsize each of the hoppers, compared to black-and-white image formation apparatuses which require only one hopper, as described above.
Moreover, since the amount of toner consumption is different for each of the colors, the supplying screws in the hoppers need to be driven separately from each other. Thus, it is inevitable that the hoppers are provided close to each other, near the motor driving the supplying screws.
Therefore, the magnetic field generated by the motor might turn on or turn off the reed switches regardless of the amount of remaining toner in the hoppers. If this is the case, an unnecessary operation for supplying toner from the toner cartridges is performed even though a sufficient amount of toner is remaining in the hoppers.
The amount of the remaining toner in each toner cartridge is estimated based on the number of the toner supply operations that have been performed. That is, the amount of the remaining toner in the toner cartridge is estimated on the assumption that a certain amount of toner is actually supplied from the toner cartridge to the hopper every time the toner supply operation is performed.
Therefore, in the case where a sufficient amount of toner is remaining in the hopper and the toner supply operation does not actually supply the hopper with toner, the amount of the remaining toner in the toner cartridge will be estimated to have been decreased even though it has not been actually decreased.
If such estimation is repeated, the apparatus displays a message saying that the toner cartridge is empty, and unnecessarily encourages the user to replace the toner cartridge with a new one.
This problem could get worse if the downsizing of image formation apparatuses progresses further and the magnetic noise source gets closer to the reed switches.